The demands placed on modern combustion engines continuously increase—both with regard to the legal parameters for allowable emission values, as well as the rising expectations of the final consumers in terms of ride comfort, running smoothness and low fuel mileage. Meeting these requirements necessitates accurately controlling the fuel combustion, in particular the combusted fuel quantity.
In the case of an internal combustion engine having what is commonly known as a common-rail system, fuel is delivered via a high-pressure pump under high pressure into a common reservoir, referred to as a common rail, and stored therein. Fuel is passed from this common rail to the injectors. The control parameters of the injectors required for the injection are specified by an engine control unit as a function of operating points. The pressure that the fuel in the common rail is under and at which the fuel is injected into the combustion chamber, is a decisive and central quantity for the combustion.
Different approaches are known for regulating this pressure, generally referred to as common-rail pressure. The regulation may be thereby carried out either on the high-pressure side via a pressure regulating valve (PRV) on the high-pressure delivery line or on the intake side (low-pressure side) by a metering unit integrated in the high-pressure pump or provided as a separate component. What are commonly known as two-actuator systems employ both design approaches. What is generally referred to as a common-rail-pressure sensor (RPS) furnishes the actual value for the regulation in each case.
The common-rail-pressure sensor is an integral component of the common-rail system. The sensor signal received via the common-rail-pressure sensor is evaluated in the engine control unit and used for adjusting the desired nominal rail pressure and for determining the electrical actuation of the injection controller, for example of a piezo injector or of an injector having a solenoid valve, that is required for a specific injected fuel quantity. An unrecognized maladjustment or drift of the common-rail-pressure sensor leads to an incorrect injected fuel quantity and thus to degraded emissions and/or to an increase in the noise generated.
Conventional common-rail-pressure sensors include a sensor element having an evaluation circuit. A raw signal received via a sensor element is preprocessed into the sensor signal by the evaluation circuit through A/D conversion, data processing and subsequent D/A conversion. If no sensor signal is available due to an error in the signal path or on the common-rail-pressure sensor itself, this is assessed, and operation of the vehicle is continued using an emergency program (in the case of common-rail systems having a pressure regulating valve), or the engine is shut off (in the case of common-rail systems that merely have a metering unit). “Breakdowns” caused in this manner are undesirable.
For these reasons, inter alia, it is expedient to monitor the common-rail pressure, respectively the common-rail-pressure sensor by checking the plausibility of the supplied sensor signal. However, this cannot be done accurately enough and within the relevant operating range using system functions. Moreover, known methods do not make possible any satisfactory measures in cases where a common-rail-pressure sensor delivers an incorrect or implausible sensor signal.
Therefore, there is a need for ways to reliably determine a common-rail pressure in the case of a common-rail system and to initiate appropriate measures in the event of an error.